1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor epitaxial wafer.
2. Description of the Related Art
Compound semiconductors have been widely used as materials for optical semiconductor elements, and a material obtained by epitaxially growing a desired layer on a single crystal substrate is used as such a semiconductor material. The reason is that a crystal used as such a substrate is difficult to use as a light emitting element as it is because it has many defects and is relatively impure. Therefore, a layer is epitaxially grown on the substrate, which has a composition such that a desired light emitting wavelength can be obtained. A ternary mixed crystal layer is primarily used as such an epitaxially grown layer. The epitaxial growth is normally in accordance with vapor-phase epitaxy or liquid-phase epitaxy.
Vapor-phase epitaxy which is normally used will be described with reference to FIGS. 1 and 3.
In vapor-phase epitaxy, epitaxial growth is achieved according to a method herein a holder 1 made of graphite or quartz is placed in a reactor 2 made of quartz and a source gas is introduced therein to be heated. On a normal holder, a substrate is positioned at an angle of several degrees to the flow of the gas.
As an example, the growth of a GaAsP epitaxial wafer on a GaP substrate will be described. A GaP substrate is a (100) plane having an orientation flat which is parallel to a cleavage plane of a (0, -1, 1) plane and which is at an angle tilted by 6.degree. in a [0, -1 -1] direction (off angle).
The substrate is normally arranged so that a [0, 1, 01] direction perpendicular to the off angle direction is at the upstream of the gas flow. In the case of GaP, the thickness of the substrate is approximately 300 .mu.m.
There has been a problem in that wafers epitaxially grown using such a method are frequently cracked and/or broken when the temperature is increased or decreased. This is because the source gas directly enters a gap between the substrate and the holder and the epitaxial layer grows at the edge on the rear side of the single crystal substrate. That is, an epitaxial layer of 50-200 .mu.m grows at the edge of the rear side of the wafer during vapor-phase epitaxy. Such an epitaxial layer frequently extends to the plane of the holder and sticks to the surface of the holder and the substrate. When the temperature is returned to the room temperature after the heating process for epitaxial growth, a crack can occur at the sticked area due to a difference in the coefficients of thermal expansion of those parts, which can result in a crack extending over the entire wafer. Further, when a small defective portion remains after epitaxial growth, such a portion has some cracks which cause the wafer to crack during processing on the outer circumference thereof after the epitaxial growth and, as a result, the yield of wafers is reduced.